Your chimney is exposed to the full force of the weather and, to ensure
the chimney can withstand such a variety of conditions, it should be checked
regularly for any potential problems.

By keeping an eye on your chimney’s condition or change of condition,
repairs can be carried out as soon as they’re necessary, preventing bigger
problems further down the line.

A combination of erosion, acid attack and salt crystallisation are
liable to cause a chimney to lean. The BRE Good Repair Guide says any chimney
that leans more than 1mm in 100mm is unsafe. This is perhaps a little too
cautious and where historic buildings are concerned, one may wish to apply the
middle third rule to the shaft wall around the flue. Thus where the shaft wall
is half a brick thick, a lean of 35mm could be tolerated, but if in any doubt,
consult a structural engineer.
Where repairs are impossible, the remedy of last resort is to rebuild the stack
from some convenient point - say the top section only or completely from below
roof level - using as much salvaged material as possible. To comply with The
Building Regulations any rebuilt section will have to have an internal diameter
of 200mm (although 185mm is acceptable for a relined flue), so some ingenuity
may be required to incorporate a flue liner within the original stack
dimensions.

If it is decided that the existing lean on a stack can be tolerated the
factors inducing the lean should be stabilised. The flue should be lined and
broken or decayed bricks should be cut out and replaced. In slender stacks it
is a wise precaution not to take out too many bricks at a time (sometimes no
more than one at a time) and the new mortar must be allowed sufficient time to
achieve strength before proceeding with the next area of cutting out. Cracks
should be repointed and all 'defective' mortar raked out and repointed.

'Defective' mortar may be taken to include that which is loose,
crumbling or cracked. As a general principle (and this applies to all
repointing, not just on chimneys) if a lime mortar is difficult to remove,
repointing is not necessary. Any hard cement mortar will be more difficult to
remove. Take care not to damage the edges of the brick or stone. Rake out to a
depth of 25mm or twice the width of the joint whichever is the greater; do not
rake out deeper than 35mm. The strength and lime content of the mortar will
depend on the hardness of the brick or stone, in accordance with usual good
practice for repointing. If it is necessary to add some cement to the mix, use
sulphate-resisting cement. Similar mixes will be used for rebuilding stacks.
Once a leaning stack has been stabilised, some periodic checking (for example
during quinquennial inspections) is advisable.